A persistent infection of the human MCF-7 cell line (MCF-7RV) was established with the DBS strain of rubella virus at a low multiplicity of infection. Fluorescent antibody staining revealed that 100% of the cells were positive for rubella antigens. The infected cells were refractory to superinfection with vesicular stomatitis virus (VSV) but were susceptible to herpes simplex virus type 2 (HSV-2). No interferon could be detected in infected cell culture fluid, and continuous passage in the presence of antibody did not lead to a decrease in the percentage of infected cells. Virus production in the persistently infected cells represented a 5- to 10-fold increase over primary infection. Plaque assays at 30 and 39 °C of the virus produced at 37 °C revealed the presence of temperature-sensitive () mutants. If MCF-7RV cells were maintained at 30 °C, significant increases in virus production were observed, leading to cytopathic effect and destruction of the monolayer. If maintained at 39 °C, MCF-7RV cells produced less virus and demonstrated normal morphology. These data suggest that the naturally selected population of mutants being produced by these cells represents the mechanism by which persistence is maintained.


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